The present invention generally relates to image output systems, and more particularly to an image output system which is suited for an outline font which describes a contour line by a mathematical formula.
Because the outline font describes a contour line by a mathematical formula, it is possible to carry out a process such as enlargement, reduction and rotation. For this reason, the outline font is now often used in place of the conventional dot font. However, the biggest problem of the outline font is that the processing speed is slow. More than half of the total processing time is spent on filling or painting the inside of the contour line.
A Japanese Published Patent Application No. 53-41017 proposes a method of increasing the painting speed. According to this proposed method, a contour line of a pattern which is displayed in dots is once written into a memory. The contour line written in the memory is successively scanned in a main scanning direction, and an interval between one black point to a next black point is filled by black for each scanning line. But according to this method, an erroneous operation occurs when the pattern includes a portion having a width of one dot. In addition, there is a problem in that the processing speed is dependent on a speed with which the contour line is written into the memory because the contour line of the pattern is once written into the memory.
An image output system is proposed in a U.S. patent application Ser. No. 489,094 filed Mar. 6, 1990 in which the assignee is the same as that of this application to prevent the above described erroneous operation and to considerably increase the processing speed. According to this image output system, coordinates of a starting point (black transition point) of a black line and a starting point (white transition point) of a white line in each scanning line are obtained and stored in an image memory. Then, the image memory is successively scanned to fill an interval between the starting point of the black line to the starting point of the white line. In other words, the starting points of the black and white lines are written as data "1" in the image memory for each scanning line. A reproduced pattern is obtained by successively scanning the image memory and setting "1" (black) from the odd numbered data "1" to the even numbered data "1" and resetting "0" (white) for the even numbered data "1" with respect to each scanning line.
According to this proposed image output system, a pair made up of a black starting point and a white starting point always exists on one scanning line even when the width of the black line corresponds to one dot, and for this reason, the pattern will not be reproduced erroneously. In addition, since the starting points of black and white lines are written into the image memory and not the contour line itself, the number of points to be written is effectively reduced thereby improving the processing speed. Normally, this proposed image output system introduces no problems. However, it was found that the following problems occur in particular combinations of patterns.
First, when a pattern such as a line graphic which requires no painting and a pattern which requires painting coexist in the image memory, it is impossible to distinguish the two patterns.
Second, when transition point information which require painting intersect in the image memory, it is impossible to distinguish the start and end of the painting.